1. Field of the Invention
The present invention relates to a substrate of a liquid crystal display device (LCD), and a method for manufacturing the same. In particular, the present invention relates to a method for manufacturing a substrate of a liquid crystal display wherein an organic layer and a pixel electrode thereon are firmly cohering to each other.
2. Description of Related Art
A conventional substrate of a liquid crystal display device having a switching element and a pixel electrode is formed, mentioned as follows, referring to FIGS. 1a and 1b. FIGS. 1a and 1b show the inverse staggered structure of the thin film transistor (TFT).
Referring to FIG. 1a, a gate electrode 60a is formed on a transparent substrate 10. A gate insulation layer 50 is deposited thereon to cover the gate electrode 60. A semiconductor layer 90 with an island shape is formed on the gate insulation layer 50 on the gate electrode 60. On the surface of the semiconductor layer 90, ohmic contact layers 92a and 92b are formed in separate regions. On one ohmic contact layer 92a, a source electrode 70a is formed and a data line 70 connecting to the source electrode 70a is formed on the gate insulation layer 50. On the other ohmic contact layer 92b, a drain electrode 70b is formed. As the gate electrode, source electrode and the drain electrode are formed, a TFT acting as a switching element is completed.
On the substrate having the TFT, an organic layer 55, including a benzocyclobutene (or BCB) generally containing combinations of C, H and O elements, is formed. A contact hole 30 is formed to expose some surface of the drain electrode 70b by patterning the organic layer 55. The reason for using the organic layer on the substrate of the LCD is that after being coated on the substrate of which surface has stepped profile, the surface of the organic layer does not project the stepped profile. That is, a substantially flat surface of the organic layer is formed. Also, the organic layer has a lower coefficient constant similar to the inorganic layer, such as silicon oxide (SiOx) or silicon nitride (SiNx). Therefore, after the organic layer is formed on the substrate, a pixel electrode can be formed over the area on which the data line is formed in order to maximize the aperture ratio. In that regard, an ITO (Indium Tin Oxide) layer is deposited on the organic layer having the contact hole 30 and is patterned to form a pixel electrode 40 by etching the ITO layer with the photo-resist 88 pattern using a wet etching method. It is possible to overlap the pixel electrode and some portion of the data line because problems such as cross talk do not occur between them due to the characteristics of the organic material mentioned above.
However, during the etching process, the ITO layer can be easily peeled off from the organic layer or a vacancy can occur between the ITO layer and the organic layer because the coherency between them is very weak. As a result, the etchant can penetrate into the vacancy so that the pixel electrode can be over etched, as shown in FIG. 1b. FIG. 1b shows the undesirable patterned results which (A) is the peeled off portion of the pixel electrode (B) and (C) are the over etched portion of the pixel electrode.
If the edge portions of the ITO layer, which constitutes the pixel electrode, are over etched by the etchant, then the size of the pixel electrode is reduced and becomes irregular. This is referred to as a CD loss. As a result, the aperture ratio becomes smaller than the expected value.